THIS invention relates to a dental implant assembly and sub-assembly.
A dental implant is an artificial tooth root replacement and is used in prosthetic dentistry to support restorations that resemble a tooth or group of teeth. Known dental implant assemblies of the osseointegrated type have three components. One of these is the implant or fixture component which takes the place of the natural tooth root and which is anchored in the jaw bone of the patient. This component is normally made of metal, typically titanium, or a ceramic, typically zirconium oxide, and has an external thread: which is engaged in a hole drilled, reamed or otherwise formed in the jaw bone. The component also has an internally threaded socket to receive a prosthetic screw as described below.
Another component of the dental implant is a dental component, typically a crown or abutment which seats on the apical or outer end of the implant component. The crown or abutment has a passage through it which aligns with the socket in the implant component. In use, it is fixed or clamped to the implant component by a component in the form of a retaining screw which passes through the passage in the abutment and is screwed into the socket.
It is desirable for the preload force applied by the screw, which serves to clamp together the fixture and crown or abutment components, to be as high as possible. It is recognised that the greater the tension in the screw, the greater the “preload” force with which the crown or abutment is clamped to the implant component. If there is an insufficient preload force the prosthetic screw may come loose, or the connection may not remain closed or may suffer from instability when functional loading, for example during mastication, is applied thereto.
Threaded advance of the screw into the socket is at least partially resisted by friction between rotating surfaces of the screw and the stationary surfaces against which the screw moves. In the prior art, it has been recognised that by reducing the frictional resistance, the preload on the screw can be increased for a given applied torque. Examples of attempts to reduce frictional resistance are described in the following documents:
U.S. Pat. No. 5,711,669—this document describes an assembly in which the screw has a relatively soft, malleable coating of gold or silver.
U.S. Pat. Nos. 5,879,161 and 6,287,116—these documents describe assemblies in which the screw is gold-plated for reduced friction.
U.S. Pat. No. 6,447,295—this document describes an assembly in which the screw is coated with a hard carbon coating or film to reduce friction. The hard carbon coating may be in the form of diamond-like carbon, amorphous diamond, crystalline diamond or a combination of such materials.
U.S. Pat. No. 7,300,283—this document describes a retaining screw fitted with a rotationally fixed spring washer made of gold to reduce friction between the head of the screw and the component against which it acts, typically the abutment.
It will be seen that in the majority of these prior art disclosures, attempts have been made to reduce friction between the rotating screw and the stationary surfaces against which it moves during the screw torqueing operation using gold or other relatively low friction metals. However experimentation by the present applicant has shown that the use of gold in or on the retaining screw or in an intervening washer provides only a modest increase in the preload force which can be obtained.